The present invention relates to radiation-curable compositions which can be used to provide solder masks in connection with printed circuit fabrication techniques, and also relates to methods for producing a cured solder mask.
As is well known in the art of printed circuit fabrication, solder masks are employed so as to prevent all but selected areas of the printed circuit (e.g., all areas other than through-holes and their surrounding pads, surface mount areas, etc.) from coming into contact with molten solder during provision of solder to these selected areas and during solder connection of various electronic components to the printed circuit at these selected areas.
Based upon considerations of ease of application and ability to obtain the fine definition usually required in most printed circuits, the most desirable solder masks are those formed from photosensitive resin compositions, i.e., compositions of organic resin materials which undergo polymerization (curing, crosslinking) under influence of radiation exposure. Typically, a layer of the photosensitive composition is deposited over the printed circuit board, heated (prebaked) to drive off a substantial portion of the solvent carrier for the composition, and then exposed to radiation of appropriate energy in a selective manner such that exposure occurs only at those areas where the solder mask is desired. The exposure brings about at least partial curing of the resin composition at such areas, rendering it there differentially less soluble (relative to a suitable developer) than at areas where the composition was not exposed. As such, unexposed composition can be selectively removed from the printed circuit board using the developer, leaving behind a distribution of the at least partially cured resin composition in the desired solder mask pattern. Suitable post-development steps, such as further radiation exposure and/or heating (postbaking) of the patterned composition, typically are then employed to effect further curing of the composition as may be needed in order for it to serve as an effective solder mask.
To best serve their intended purpose, solder masks should exhibit a high degree of chemical resistance to solder and the fluxes used in soldering operations, and a high degree of thermal resistance relative to the elevated temperatures used in soldering operations. Many solder masks produced from photosensitive resin compositions are deficient in these regards, having tendencies to degrade, blister or separate from the circuit board under conditions of soldering applications. Yet other photosensitive resin compositions produce solder masks which are excessively brittle, with increased tendency to chipping and flaking under conditions normally encountered in handling and processing of the circuit boards on which they are arranged. Attempts at solving these problems often tend to be counter-productive, i.e., generating problems associated with the resin composition itself, such as premature curing, instability, short shelf life, and the like.
The formulation of a suitable photosensitive resin composition for production of a solder mask thus involves a careful balancing of factors, often seemingly inconsistent, so as to attain desirable properties in the composition itself and in a solder mask produced from it.